Disallow a specific function template instantiation

Question

I would like to define a template function but disallow instantiation with a particular type. Note that in general all types are allowed and the generic template works, I just w

Answer 1

You could use a functor instead of a function:

template
struct convert { 
    T operator()(char const * in) const { return T(); } 
};
template<> struct convert;

int main()
{
    char const * str = "1234";

    int a = convert()( str );
    double b = convert()( str ); // error in this line

    return 0;
}

This will give you an error at the point of instantiation.

By adding helper function you will get the wanted behaviour:

template
struct convert_helper { 
    T operator()(char const * in) const { return T(); } 
};
template<> struct convert_helper;

template
T convert( char const * in ) { return convert_helper()( in ); }

int main()
{
    char const * str = "1234";

    int a = convert( str );
    double b = convert( str );

    return 0;
}